1. Field of the Invention
This invention relates to an apparatus for protecting switching circuits having parallel multiple switching elements with the characteristic of self-firing due to an over-voltage and a method of protecting the switching circuits.
2. Description of the Prior Art
Over-voltages are generated in the field windings of generators and motors according to asynchronous inputs, short circuits, and the like. Over-voltage protection circuits, in which switching elements, for example thyristors, are applied to the protection of these devices from over-voltages. FIGS. 3 and 4 show prior art over-voltage protection circuits having thyristors as switching elements.
In FIG. 3, 41 is a generator, 42 is a field winding of generator 41, 43 is a thyristor rectifier which supplies direct current (DC) power to the field winding 42 and 44 is an over-voltage protection circuit in which thyristors 45 are used for protecting from over-voltages generated in the field winding 42.
The thyristors 45 in over-voltage protection circuit 44 are caused to fire by over-voltages generated in the field coil 42 and form a short circuit, thus protecting the generator 41 and the thyristor rectifier 43. In order to improve the reliability of this over-voltage protection circuit, thyristors 45 are used which have the characteristic of self-firing without a gate signal when a certain level of voltage is impressed (hereafter, these thyristors 45 are referred to as VBO free thyristors).
FIG. 4 is a detailed diagram of a prior art over-voltage protection circuit which uses VBO free thyristors. Although a suitable number of VBO free thyristors may be connected in series or parallel according to the voltage and current used in the over-voltage protection circuit, FIG. 4 shows the case of 3 parallels. Elements 10, 20 and 30 are reactors which equalize the current shares between the parallel circuits. Elements 11, 12, 21, 22, 31 and 32 are VBO free thyristors. Element 55 is a gate circuit which supplies gate pulses to VBO free thyristors 11, 12, 21 ,22 ,31 and 32 when an ON command is supplied to the over-voltage protection circuit.
In the circuit in FIG. 4, VBO free thyristors 11, 12, 21, 22, 31 and 32 can be switched ON by detecting faults such as circuit over-voltage or generator short-circuits. However, even when there is no ON command, VBO free thyristors 11, 12, 21, 22, 31 and 32 can fire by themselves and prevent over-voltage.
In the prior art circuit in FIG. 4, when VBO free thyristors 11, 12, 21, 22, 31 and 32 self-fire due to over-voltage, they do not fire simultaneously due to randomness of firing voltage levels between individual thyristors and temperature variations. If it is assumed that the firing level of VBO free thyristor 11 is the lowest, VBO free thyristor 11 will fire first. The voltage impressed on VBO free thyristor 12 will increase due to the firing of VBO free thyristor 11. Therefore, VBO free thyristor 12 will also fire almost simultaneously. In this way, the series circuit which includes VBO free thyristors 11 and 12 will fire.
On the other hand, when the self-firing levels of the series circuits which include VBO free thyristors 21, 22, 31 and 32 are high, over-voltage is inhibited due to the firing of the series circuit which includes VBO free thyristors 11 and 12. Therefore, the series circuits which include VBO free thyristors 21, 22, 31 and 32 will not fire. For this reason, the circuit current concentrates in one of the three parallel circuits, so that three times the current flows through VBO free thyristors 11 and 12, and VBO free thyristors 11 and 12 will break down.
As described above, due to randomness between the self-firing voltages of individual VBO free thyristors and temperature variations, the flow of current becomes concentrated in one of the parallel circuits. In order to prevent this, devices with completely identical characteristics were selected from a suitable number of VBO free thyristors and these were connected in parallel. In addition, temperature control for the unit was required in order to prevent the temperatures from becoming uneven. Thus, there was the problem of the unit becoming bulky and also very costly.